Measurement of Natural Losses of LNAPL Using CO2 Traps

Efflux of CO₂ above releases of petroleum light nonaqueous phase liquids (LNAPLs) has emerged as a critical parameter for resolving natural losses of LNAPLs and managing LNAPL sites. Current approaches for resolving CO₂ efflux include gradient, flux chamber, and mass balance methods. Herein a new method for measuring CO₂ efflux above LNAPL bodies, referred to as CO₂ traps, is introduced. CO₂ traps involve an upper and a lower solid phase sorbent elements that convert CO₂ gas into solid phase carbonates. The sorbent is placed in an open vertical section of 10 cm ID polyvinyl chloride (PVC) pipe located at grade. The lower sorbent element captures CO₂ released from the subsurface via diffusion and advection. The upper sorbent element prevents atmospheric CO₂ from reaching the lower sorbent element. CO₂ traps provide integral measurement of CO₂ efflux based over the period of deployment, typically 2 to 4 weeks. Favorable attributes of CO₂ traps include simplicity, generation of integral (time averaged) measurement, and a simple means of capturing CO₂ for carbon isotope analysis. Results from open and closed laboratory experiments indicate that CO₂ traps quantitatively capture CO₂. Results from the deployment of 23 CO₂ traps at a former refinery indicate natural loss rates of LNAPL (measured in the fall, likely concurrent with high soil temperatures and consequently high degradation rates) ranging from 13,400 to 130,000 liters per hectare per year (L/Ha/year). A set of field triplicates indicates a coefficient of variation of 18% (resulting from local spatial variations and issues with measurement accuracy).